The present invention relates to printing of patterns with extremely high precision on photosensitive surfaces, such as photomasks for semiconductor devices and displays. It also relates to direct writing of semiconductor device patterns, display panels, integrated optical devices and electronic interconnect structures. Furthermore, it can have applications to other types of precision printing such as security printing. The term printing should be understood in a broad sense, meaning exposure of photoresist and photographic emulsion, but also the action of light on other light sensitive media such as dry-process paper, by ablation or chemical processes activated by light or heat. Light is in this application limited to mean extreme ultraviolet (EUV). EUV is in this application defined as the range from 100 nm and down as far as the radiation is possible to treat as light. A typical wavelength for EUV is 13 nm.
In a different sense the invention relates to the art and science of spatial light modulators and projection displays and printers using such modulators. In particular it improves the grey-scale properties, the image stability through focus and image uniformity and the data processing for such modulators by application of analog modulation technique. The most important use of the analog modulation is to generate an image in a high-contrast material such as photoresist with an address grid, i.e. the increment by which the position of an edge in the pattern is specified, that is much finer than the grid created by the pixels of the spatial light modulator.
It is known in the current art to build precision pattern generators using projection of micromirror spatial light modulators (SLMs) of the micromirror type (Nelson 1988, Kxc3xcck 1990). To use an SLM in a pattern generator has a number of advantages compared to the more wide-spread method of using scanning laser spots: the SLM is a massively parallel device and the number of pixels that can be written per second is extremely high. The optical system is also simpler in the sense that the illumination of the SLM is non-critical, while in a laser scanner the entire beam path has to be built with high precision. Compared to some types of scanners, in particular electrooptic and acoustooptic ones, the micromirror SLM can be used at shorter wavelengths since it is a purely reflective device.
In both references cited above the spatial modulator uses only on-off modulation at each pixel. The input data is converted to a pixel map with one bit depth, i.e. with the values 0 and 1 in each pixel. The conversion can be done effectively using graphic processors or custom logic with area fill instructions.
In a previous application by the same inventor Sandstrxc3x6m (Sandstrxc3x6m et. al. 1990), the ability to use an intermediate exposure value at the boundary of a pattern element to fine-adjust the position of the element""s edge in the image created by a laser scanner was described.
It is also known in the art to create a grey-scale image, preferably for projection display of video images and for printing, with an SLM by variation of the time a pixel is turned on or by printing the same pixel several times with the pixel turned on a varying number of times. The present invention devices a system for direct grey-scale generation with a spatial light modulator, with a special view to the generation of ultra-precision patterns. Important aspects in the preferred embodiments, are uniformity of the image from pixel to pixel and independence of exact placement of a feature relative to the pixels of the SLM and stability when focus is changed, either with intention or inadvertently.
Specifically, in conventional pattern generators EUV could not be used for illumination. This is due to the fact that EUV requires reflective optics. Furthermore, EUV has a very small wavelength, and could thus be used for printing very detailed patterns. However, this requires a very high data rate, which is not possible to obtain with the conventional technique.
Optical pattern generators for microlithography typically uses light in the wavelength range 350-450 nm, and has a limited optical resolution. A new generation of pattern generators using deep UV, i.e. wavelengths around 250 nm, are in development but not commercially available. This new generation of equipment will approximately double the optical resolution, but the laws of physics and the availability of suitable optical materials and light sources makes it difficult to increase the resolution any further.
It is therefore an object of the present invention to provide a pattern generator using EUV for the illumination.
This object is achieved with an apparatus according to the appended claims.
The apparatus according to the invention for creating a pattern with extremely high resolution on a workpiece, such as a pattern on a semiconductor chip with 50 nm wide lines, comprises
a source for emitting electromagnetic radiation in the EUV wavelength range,
a spatial modulator having a multitude of modulating elements (pixels), adapted to being illuminated by said radiation
a projection system creating an image of the modulator on the workpiece,
an electronic data processing and delivery system receiving a digital description of the pattern to be written, extracting from it a sequence of partial patterns, converting said partial patterns to modulator signals, and feeding said signals to the modulator, a precision mechanical system for moving said workpiece and/or projection system relative to eachother an electronic control system coordinating the movement of the workpiece, the feeding of the signals to the modulator and the intensity of the radiation, so that said pattern is stitched together from the partial images created by the sequence of partial patterns.
By using an SLM as the modulator it is possible to use EUV. With such an apparatus very small features in the pattern could be written. The unique properties of spatial light modulators, i.e. entirely reflective light paths and the possibility of achieving an extremely high data rate, makes it possible to build a pattern generator using EUV and obtain substantially higher optical resolution than known technology, and at the same time achieve practical writing speeds. Previously known technology, including optical scanning, focused particle beams or X-ray, fail to achieve the same combination of performance and throughput.